1. Field of the Invention
The present invention relates to a method of and a device for determining a time-dependent gradient of a shock wave in a percussion or subjected to a percussion load ferromagnetic element and including subjecting the ferromagnetic element to action of a magnetic flux, and providing voltage measuring means associated with the ferromagnetic element for determining a change of a magnetic flux velocity, which is determined as a change in a measuring voltage, during a percussion action
2. Description of the Prior Art
The knowledge of a time-dependent gradient of a shock wave(s) in an element, in particular in a percussion tool, is necessary for optimization of technical characteristics, for testing, for programming of the drive, and for quality control of percussion tools used in construction such as, e.g., hammer drills. For measuring the shock waves, at present, wire strain gauges are almost exclusively used. The mounting of as a such wire strain gauge on a scale or on a tool shaft is very expensive. In addition, in rotatable tools, the measurement is possible only by using an expensive transfer electronics. In addition, wire strain gauges, in particular their electrical contacts, are very sensitive to vibrations.
For measuring static or quasi static forces, sensors, which are based on a magnetoelastic principle, present a possible alternative to wire strain gauges (see Jarosevic, A. et al., Vorspannungsmessung an Baukonstruktionen (Prestress Measurements in Construction), Braunschweiger Constructional Seminar, Braunschweig, Nov. 12-13, 1992. Vol. 97, p.p. 71-82. The magnetoelastic effect (see Seekircher, S., Magnetoelastiche Kraftsensoren mit Amorphen Metallen (Magnetoeleastic force sensors with amorphous metals), VDI Progress Reports, Series 8 Mess-, Steuerungs and Regelungstechnik (Measuring, Control and Regulating Technique), No. 266, VD1 Publishing House; and Boll, R., Weichmagnetische Werkstoffe (soft magnetic materials), 4 edition, ISBN 3-8009-1548-4, Vacuum-Schmelze GMBH, 1990) and reversal of magnetostriction represent a change of magnetic characteristics, e.g., of permeability xcexcr under a mechanical load. In the latter case, an induced magnetic flux in a ferromagnetic material is changed dependent on the applied mechanical stress. FIG. 1 of the drawings shows qualitatively the influence of a compression stress on a conventional magnetizing-hysteresis curve. For measuring of a mechanical stress applied to a ferromagnetic element, the loaded element 1 (see FIG. 2) is magnetized with a magnetizing coil 2 (see FIG. 2) to which an alternating current I is fed, with an alternating field being formed in the magnetizing coil. The generated alternating magnetic flux produces in the second coil, the measuring coil 3, an electrical voltage proportional to the velocity change of the magnetic flux. From the amplitude ratio of the measuring voltage U to the current I in the magnetizing coil 2, a mechanical stress and, thereby, the load, which is applied to the element 1, can be calculated.
For measuring of rapidly changing mechanical stresses or strains, which occurs at shock waves, the magnetization can be effected with a constant field (see Hecker, R., Schrxc3x6der, P. Nutzung mechanische u. electromechanische effecte zur messung Elastischer Wellen in Staben (The use of mechanical and Electromechanical Effects for Measuring Elastic Waves in Bars), Technical measurements, v. 11/95, R. Oldenburg Publishing House, and Hecker R., Anwendung des magnetoelastischen effecte zur messung von Dennwellen in stabformigen korpen schlagender machinen (The Use of the Magnetoelastic Effect for Measuring Extensional Waves in Bodies of Percussion Tool), Technical Measurements v. 6/88, 55, 1988.
The measuring coil 3 permits to obtain the speed change of the mechanical stress, i.e., a mathematical expression of the first derivative of the shock wave. By a simple integration, the stress gradient of the shock wave is obtained in element 4 (FIG. 2.).
However, very rapid changes of the magnetic flux, upon propagation of a shock wave in element 1, induce in the electrically conductive material of the element 1, e.g., in a drill, eddy currents. These eddy currents influence the magnetic flux and, thereby, the gradient of the voltage U at the terminals of the measuring coil 3. This effect is described in publication of the firm HILTI AG, Schaan 1994, an assignee herein, Malkinsky, L. M., On Magnetoelastic Sensor Design for the Impact Energy Measurement, Report A-IF7-45/94. Because the eddy currents are spread primarily in the outer surface region of the element 1, due to the skin-effect, it is proposed to reduce the eddy current effect on the tool surface by providing notches therein.
For measuring mechanical stresses in the known ferromagnetric elements, it is known to glue onto a loaded element strips of materials with particular distinctive electroelastic properties or characteristics (e.g., of amorphous metals), and to measure changes of the magnetic flux in these strips. The drawback of this method consist in that, as with the use of wire strain gauges, an expensive gluing or a similar process is necessary.
Accordingly, an object of the present invention is to provide a method of and a device for determining of a time-dependent gradient of a shock wave in an element subjected to percussive loads, e.g., in a percussion tool.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a method according to which the ferromagnetic element is subjected to action of a magnetic flux voltage, there are provided measuring means associated with the ferromagnetic element for determining a change of a magnetic flux velocity, which is determined as a change in a measuring voltage, during a percussion action, and the gradient of the shock wave is determined by additive superimposition of the measuring voltage and integrated measuring voltage. A device according to the present invention includes a magnetizing coil surrounding the ferromagnetic element, means for energizing the magnetizing coil and including a controlled current source, with regulating voltage being used as a measuring voltage, or an inductance seriesly connected with the voltage source, with the measuring voltage being tapped on the series inductance, and means for determining the shock wave gradient by additive superimposition of the measuring voltage and integrated measuring voltage.
As discussed above, up to the present, it was not possible to exactly measure the shock wave gradient by using magnetoelastic sensors because of eddy currents induced in the element in question (see Hecker, R., The use of the Magneto elastic Effect . . . ).
The present invention is based on a premise that it should be possible to describe the effect of the eddy currents on the shock wave gradient by using a mathematical model or an equivalent circuit. It was based on the observation that eddy current counteract to the changes of the magnetic flux. This results in that the magnetic flux with an increasing frequency is not any more proportional to the mechanical strains in the element, but rather contain an ever increasing integral portion thereof. In other words, it was observed that the measuring voltage is not any more a pure differential of the shock wave, but rather, with an increase of frequency, contains an increased proportional part thereof.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to the construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.